Porous ceramic structure and method of producing porous ceramic structure

ABSTRACT

When the porous ceramic structure contains Co together with Fe or Mn, the Co content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Co 3 O 4 , and when the porous ceramic structure contains Co without containing Fe and Mn, the Co content is higher than or equal to 0.2 mass % and lower than or equal to 6.0 mass % in terms of Co 3 O 4 . The Ce content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO 2 . The Fe/Mn/Co ratio is higher than or equal to 0.8 and lower than or equal to 9.5. The content of the metal oxide particles is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.

TECHNICAL FIELD

The present invention relates to a porous ceramic structure and a method of producing a porous ceramic structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority benefit under 35 U.S.C. Section 119 of Japanese Patent Application No. 2020-57683 filed in the Japan Patent Office on Mar. 27, 2020, the entire disclosure of which is incorporated herein by reference.

BACKGROUND ART

Japanese Patent Application Laid-Open No. 2017-186220 (Document 1) proposes cerium dioxide particles that contain therein or thereon a transition-metal oxide containing iron and manganese. It is assumed that such cerium dioxide particles are, for example, used as an oxidation catalyst in a diesel particulate filter (DPF) that includes a diesel oxidation catalyst (DOC) and a catalyzed soot filter (CSF).

Japanese Patent Application Laid-Open No. 2018-30105 (Document 2) and Japanese Patent Application Laid-Open No. 2017-171543 (Document 3) propose techniques that allow a porous ceramic structure used in a DPF or other measures to support a sufficient amount of a catalyst in order to maintain a catalytic activity. In the porous ceramic structure, portions of cerium dioxide particles are taken into the structure and the other portions of the ceramic diode particles are exposed to the surfaces of pores in the structure. In the porous ceramic structure according to Document 2, portions of cerium dioxide particles that are exposed to the surfaces of pores contain an iron oxide. In the porous ceramic structure according to Document 3, portions of cerium dioxide particles that are exposed to the surfaces of pores support fine catalyst particles of an element of the platinum group.

The porous ceramic structures used in DPF or other measures are required to achieve both a reduction in pressure loss and an improvement in catalytic performance.

SUMMARY OF INVENTION

The present invention is directed to a porous ceramic structure, and it is an object of the present invention to provide a porous ceramic structure with low pressure loss and high catalytic performance.

A porous ceramic structure according to a preferable embodiment of the present invention includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt. The metal oxide particles include a fixedly attached portion located inside the structure body, and a protrusion contiguous with the fixedly attached portion and protruding into the pore. When the porous ceramic structure contains iron, an iron content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, when the porous ceramic structure contains manganese, a manganese content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃, when the porous ceramic structure contains cobalt together with iron or manganese, a cobalt content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, when the porous ceramic structure contains cobalt without containing iron and manganese, the cobalt content is higher than or equal to 0.2 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄, and a cerium content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO₂. A ratio of a sum of the iron content in terms of Fe₂O₃, the manganese content in terms of Mn₂O₃, and the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 0.8 and lower than or equal to 9.5. A content of the metal oxide particles is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.

In other words, a porous ceramic structure according to another preferable embodiment of the present invention includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least iron or manganese among iron, manganese, and cobalt. The metal oxide particles include a fixedly attached portion located inside the structure body, and a protrusion contiguous with the fixedly attached portion and protruding into the pore. When the porous ceramic structure contains iron, an iron content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, when the porous ceramic structure contains manganese, a manganese content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃, when the porous ceramic structure contains cobalt, a cobalt content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, and a cerium content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO₂. A ratio of a sum of the iron content in terms of Fe₂O₃, the manganese content in terms of Mn₂O₃, and the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 0.8 and lower than or equal to 9.5. A content of the metal oxide particles is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.

A porous ceramic structure according to another preferable embodiment of the present invention includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing cobalt and not containing iron and manganese. The metal oxide particles include a fixedly attached portion located inside the structure body, and a protrusion contiguous with the fixedly attached portion and protruding into the pore. In the porous ceramic structure, a cobalt content is higher than or equal to 0.2 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄, and a cerium content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO₂. A ratio of a sum of the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 0.8 and lower than or equal to 9.5. A content of the metal oxide particles is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.

According to the present invention, it is possible to provide a porous ceramic structure with low pressure loss and high catalytic performance.

Preferably, when the porous ceramic structure contains iron, the iron content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, when the porous ceramic structure contains manganese, the manganese content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃,

when the porous ceramic structure contains cobalt together with iron or manganese, the cobalt content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, when the porous ceramic structure contains cobalt without containing iron and manganese, the cobalt content is higher than or equal to 3.0 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄, and the cerium content is higher than or equal to 1.5 mass % and lower than or equal to 4.5 mass % in terms of CeO₂. The ratio of the sum of the iron content in terms of Fe₂O₃, the manganese content in terms of Mn₂O₃, and the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 1.0 and lower than or equal to 4.0.

Preferably, the metal oxide particles are particles of an oxide that has a spinel structure containing iron, manganese, and oxygen, or particles of an oxide that has a spinel structure containing cobalt and oxygen.

Preferably, the metal oxide particles have a mean particle diameter greater than or equal to 10 nm and less than or equal to 1 μm.

Preferably, the above-described porous ceramic structure is for use in a diesel particulate filter that collects particulate matter in an exhaust gas emitted from a diesel engine.

The present invention is also directed to a method of producing a porous ceramic structure. A method of producing a porous ceramic structure according to a preferable embodiment of the present invention includes a) preparing kneaded clay by kneading a raw material, b) obtaining a compact by molding the kneaded clay, and c) firing the compact. The raw material includes cordierite, cerium, and at least one of iron, manganese, and cobalt. The operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt. The operation c) includes c1) raising a temperature of the compact to a first temperature that is lower than or equal to a liquid-phase formation temperature, c2) raising the temperature of the compact from the first temperature to a second temperature that is higher than the liquid-phase formation temperature and lower than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the compact from the second temperature to a third temperature that is higher than the crystallization temperature and lower than or equal to a maximum temperature, and c4) maintaining the temperature of the compact at the maximum temperature for a predetermined period of time. A rate of raising the temperature in the operation c2) is higher than or equal to 100° C./h and lower than or equal to 200° C./h.

Preferably, a rate of raising the temperature in the operation c3) is higher than or equal to 100° C./h and lower than or equal to 200° C./h.

Preferably, in the operation c3), the temperature of the compact is maintained at the crystallization temperature for a predetermined period of time.

A method of producing a porous ceramic structure according to another preferable embodiment of the present invention includes a) preparing kneaded clay by kneading a raw material, b) obtaining a compact by molding the kneaded clay, and c) firing the compact. The raw material includes cordierite, cerium, and at least one of iron, manganese, and cobalt. The operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt. The operation c) includes c1) raising a temperature of the compact to a first temperature that is lower than or equal to a liquid-phase formation temperature, c2) raising the temperature of the compact from the first temperature to a second temperature that is higher than the liquid-phase formation temperature and lower than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the compact from the second temperature to a third temperature that is higher than the crystallization temperature and lower than or equal to a maximum temperature, and c4) maintaining the temperature of the compact at the maximum temperature for a predetermined period of time. A rate of raising the temperature in the operation c2) is higher than or equal to 1° C./h and lower than or equal to 10° C./h.

Preferably, in the operation c3), the temperature of the compact is maintained at the crystallization temperature for a predetermined period of time.

A method of producing a porous ceramic structure according to yet another preferable embodiment of the present invention includes a) preparing kneaded clay by kneading a raw material, b) obtaining a compact by molding the kneaded clay, and c) firing the compact. The raw material includes cordierite, cerium, and at least one of iron, manganese, and cobalt. The operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to the structure body, and metal oxide particles each fixedly attached to an inside of a pore in the structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt. The operation c) includes c1) raising a temperature of the compact to a first temperature that is lower than or equal to a liquid-phase formation temperature, c2) raising the temperature of the compact from the first temperature to a second temperature that is higher than the liquid-phase formation temperature and lower than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the compact from the second temperature to a third temperature that is higher than the crystallization temperature and lower than or equal to a maximum temperature, and c4) maintaining the temperature of the compact at the maximum temperature for a predetermined period of time. In the operation c3), the temperature of the compact is maintained at the crystallization temperature for a predetermined period of time.

These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an exhaust gas purification system;

FIG. 2 is a diagram illustrating a porous ceramic structure;

FIG. 3 is a sectional view of the porous ceramic structure;

FIG. 4 is a diagram illustrating part of a partition wall in enlarged dimension;

FIG. 5 shows an SEM image of the surface of a pore in a honeycomb structure;

FIG. 6 shows an enlarged SEM image of metal oxide particles on the surface of a pore;

FIG. 7 is a sectional view of an area in the vicinity of a metal oxide particle;

FIG. 8 is a diagram illustrating a procedure of a method of producing a porous ceramic structure;

FIG. 9 is a diagram illustrating a procedure of the method of producing a porous ceramic structure;

FIG. 10 is a diagram showing an example of a temperature profile during a firing step;

FIG. 11 is a diagram showing an example of the temperature profile during the firing step;

FIG. 12 is a diagram showing an example of the temperature profile during the firing step;

FIG. 13 is a diagram showing an example of the temperature profile during the firing step;

FIG. 14 is a diagram showing an example of the temperature profile during the firing step;

FIG. 15 shows an SEM image of the surface of a porous ceramic structure according to Example 3;

FIG. 16 shows an enlarged SEM image of the surface of a pore in the porous ceramic structure according to Example 3;

FIG. 17 shows an SEM image of the surface of a porous ceramic structure according to Example 4; and

FIG. 18 shows an enlarged SEM image of the surface of a pore in the porous ceramic structure according to Example 4.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a diagram illustrating a configuration of an exhaust gas purification system 8. The exhaust gas purification system 8 is configured to purify an exhaust gas emitted from an engine. The exhaust gas purification system 8 includes a diesel particulate filter (DPF) 81, a selective catalytic reduction (SCR) catalytic converter 85, and an urea injector 86. The DPF 81, the urea injector 86, and the SCR catalytic converter 85 are arranged in this order in the direction of flow of the exhaust gas.

The DPF 81 includes a diesel oxidation catalyst (DOC) 82 and a catalyzed soot filter (CSF) 83. The DOC 82 includes a honeycomb structure whose interior is partitioned into a plurality of cells by a partition wall, and a precious-metal oxidation catalyst supported by the partition wall. The CSF 83 includes a honeycomb structure similar to that described above and a metal oxidation catalyst supported by a partition wall of the honeycomb structure. The details of the structure of the CSF 83 will be described later. The urea injector 86 is provided in a path of the exhaust gas between the DPF 81 and the SCR catalytic converter 85. The SCR catalytic converter 85 includes a honeycomb structure similar to that described above and an SCR catalyst supported by a partition wall of the honeycomb structure.

The exhaust gas emitted from the engine flows into the DOC 82 of the DPF 81. The exhaust gas contains nitrogen monoxide (NO), oxygen (O₂), and nitrogen (N₂) and causes reactions expressed by Equations 1 and 2 below in the DOC 82. The reaction expressed by Equation 1 generates nitrogen dioxide (NO₂). Note that SOF (soluble organic fraction) in Equation 2 below is contained in particulate matter (PM) in the exhaust gas. 2NO+O₂=2NO₂  (Equation 1) SOF+O₂=CO,CO₂,H₂O  (Equation 2)

The CSF 83 collects carbon (soot) contained in the exhaust gas. In the CSF 83, the soot and NO₂ cause reactions (combustion reactions) expressed by Equations 3, 4, and 5 below, and NO is generated from NO₂. C(soot)+2NO₂=CO₂+2NO  (Equation 3) C(soot)+NO₂=CO+NO  (Equation 4) C(soot)+½O₂+NO₂=CO₂+NO  (Equation 5)

The urea injector 86 mixes urea into the exhaust gas emitted from the CSF 83, and an exhaust gas containing ammonia (NH₃) generated by decomposition of the urea flows into the SCR catalytic converter 85. In the SCR catalytic converter 85, reactions expressed by Equations 6, 7, and 8 below occur, so that NOx contained in the exhaust gas is purified. 4NO+4NH₃+O₂=4N₂+6H₂O  (Equation 6) NO+NO₂+2NH₃=2N₂+3H₂O  (Equation 7) 6NO₂+8NH₃=7N₂+12H₂O  (Equation 8)

The reaction expressed by Equation 7 is called a Fast SCR reaction and proceeds at a higher reaction rate than the reactions expressed by Equations 6 and 8. In order to improve the efficiency of the reactions occurring in the SCR catalytic converter 85 in accordance with Equation 7, the ratio between the amounts of substances NO and NO₂, which flow into the SCR catalytic converter 85, is required to be 1:1. Meanwhile, the CSF 83 consumes a large amount of NO₂ in the combustion of soot and generates NO as expressed by Equations 3, 4, and 5 described previously.

In view of this, the exhaust gas purification system 8 according to the present invention includes, as the CSF 83, a porous ceramic structure (described later) that includes an oxidation catalyst. The porous ceramic structure oxidizes part of NO to generate NO₂, i.e., converts NO into NO₂. This allows the ratio between the amounts of substances NO and NO₂, which flow into the SCR catalytic converter 85, to approach 1:1 and thereby improves the efficiency of the reactions occurring in the SCR catalytic converter 85.

In the case where a certain amount or more of soot is deposited on the CSF 83, the exhaust gas purification system 8 performs processing for burning the soot (i.e., regeneration). In this case as well, the reactions (combustion reactions) expressed by Equations 3, 4, and 5 occur in the CSF 83. If a large amount of carbon monoxide (CO) generated by these reactions flows into the SCR catalytic converter 85, the NOx purification efficiency of the SCR catalytic converter 85 may decline. The same applies to the case where a large amount of hydrocarbon (HC) contained in the fuel supplied to the CSF 83 flows into the SCR catalytic converter 85 during the processing for burning the soot.

In the exhaust gas purification system 8 according to the present invention, since the porous ceramic structure including the aforementioned oxidation catalyst is provided as the CSF 83, part of CO is oxidized into carbon dioxide (CO₂), and part of HC is oxidized into CO₂ and H₂O. This suppresses the flow of CO, HC, and other substances into the SCR catalytic converter 85 and suppresses a decline in the NOx purification efficiency of the SCR catalytic converter 85.

FIGS. 2 and 3 are diagrams illustrating a porous ceramic structure 1 in simplified form. The porous ceramic structure 1 is a tubular member that is long in one direction, and FIG. 2 illustrates the end face on one side of the porous ceramic structure 1 in the longitudinal direction. FIG. 3 is a sectional view of the porous ceramic structure 1 and illustrates part of a section taken along the longitudinal direction of the porous ceramic structure 1.

The porous ceramic structure 1 includes a honeycomb structure 10 serving as a porous structure body, cerium-containing particles, and metal oxide particles serving as an oxidation catalyst. The honeycomb structure 10 includes a tubular outer wall 11 and a partition wall 12. The tubular outer wall 11 has a tubular shape extending in the longitudinal direction. A sectional shape of the tubular outer wall 11 that is perpendicular to the longitudinal direction may, for example, be circular, or may be polygonal or any other shape. The partition wall 12 is provided in the interior of the tubular outer wall 11 and partitions the interior into a plurality of cells 13. The honeycomb structure 10 is a cell structure whose interior is partitioned into a plurality of cells 13 by the partition wall 12. The tubular outer wall 11 and the partition wall 12 are made of a porous material. As will be described later, the exhaust gas passes through pores of the partition wall 12. In order to increase the strength of the porous ceramic structure 1, the partition wall 12 has, for example, a thickness greater than or equal to 50 micrometers (μm), preferably greater than or equal to 100 μm, and more preferably greater than or equal to 150 μm. In order to reduce pressure loss in the partition wall 12, the partition wall 12 has, for example, a thickness less than or equal to 500 μm and preferably less than or equal to 450 μm.

Each cell 13 is a space extending in the longitudinal direction. Sectional shapes of the cells 13 that are perpendicular to the longitudinal direction may, for example, be polygonal (e.g., triangular, quadrangular, pentagonal, or hexagonal), or may be circular or any other shape. The cells 13 typically have the same sectional shape. Alternatively, the cells 13 may include cells 13 having different sectional shapes. In order to improve oxidation performance of the porous ceramic structure 1, the density of the cells (cell density) is, for example, higher than or equal to 8 cells per square centimeters (cells/cm²) and preferably higher than or equal to 15 cells/cm². In order to reduce pressure loss, the cell density is, for example, lower than or equal to 95 cells/cm′ and preferably lower than or equal to 78 cells/cm′.

In the porous ceramic structure 1 used in the CSF 83, the exhaust gas flows from the DOC 82, with one end in the longitudinal direction of the honeycomb structure 10 as an inlet and the other end thereof as an outlet. A predetermined number of cells 13 each have a sealer 14 provided at the inlet-side end, and the remaining cells 13 each have a sealer 14 provided at the outlet-side end. Therefore, the exhaust gas flowing into the honeycomb structure 10 flows from the cells 13 whose inlet sides are not sealed to the cells 13 whose outlet sides are not sealed, while passing through the partition wall 12 (see arrows A1 in FIG. 3 ). At this time, the exhaust gas is oxidized by the metal oxide particles (i.e., oxidation catalyst) on the partition wall 12. At each of the inlet- and outlet-side ends of the honeycomb structure 10, it is preferable that the sealers 14 are alternately provided in the direction of arrangement of the cells 13.

The honeycomb structure 10 is composed primarily of cordierite. The honeycomb structure 10 may be composed of only cordierite, or may contain materials other than cordierite (e.g., metal or ceramic other than cordierite). The cordierite content in the honeycomb structure 10 is, for example, higher than or equal to 75 mass % and preferably higher than or equal to 80 mass %. In the present embodiment, the honeycomb structure 10 is substantially composed of only cordierite.

FIG. 4 is a diagram illustrating part of the partition wall 12 in the porous ceramic structure 1 in enlarged dimension. The honeycomb structure 10 has a large number of gas cavities (hereinafter, also referred to as “pores 121”). The metal oxide particles 2 and the cerium-containing particles 3 described above are fixedly attached to the insides of the pores 121 (i.e., the surface of the pores) in the honeycomb structure 10. FIG. 4 schematically illustrates metal oxide particles 2 and cerium-containing particles 3 on the surfaces of pores 121 by cross hatching without distinguishing between them. Note that the metal oxide particles 2 and the cerium-containing particles 3 do not necessarily have to cover the entire surfaces of the pores 121.

In order to reduce pressure loss in the porous ceramic structure 1, the partition wall 12 in the honeycomb structure 10 has, for example, an open porosity higher than or equal to 25%, preferably higher than or equal to 30%, and more preferably higher than or equal to 35%. From the viewpoint of ensuring the strength of the porous ceramic structure 1, the open porosity of the partition wall 12 is, for example, lower than or equal to 70% and preferably lower than or equal to 65%. The open porosity may be measured by, for example, the Archimedes method using deionized water as a medium.

The partition wall 12 in the honeycomb structure 10 has, for example, a mean particle diameter greater than or equal to 5 μm and preferably greater than or equal to 8 μm. As in the case of the open porosity, pressure loss in the porous ceramic structure 1 decreases as the mean pore diameter of the partition wall 12 increases. In order to improve the oxidation performance of the porous ceramic structure 1, the mean particle diameter of the honeycomb structure 10 is, for example, less than or equal to 40 μm, preferably less than or equal 30 μm, and more preferably less than or equal to 25 μm. The mean pore diameter may be measured by, for example, mercury injection (in accordance with JIS R1655). Depending on the design of the porous ceramic structure 1, the sealers 14 may be omitted, and the metal oxide particles 2 may be held in a layer on the surfaces of the cells 13.

FIG. 5 shows a scanning electron microscope (SEM) image of the surface of a pore 121 in the honeycomb structure 10. In FIG. 5 , a large number of metal oxide particles 2 having relatively small particle diameters and a large number of cerium-containing particles 3 having particle diameters greater than the particle diameters of the metal oxide particles 2 are fixedly attached to the surface of the pore 121. For example, the metal oxide particles 2 have a mean particle diameter greater than or equal to 10 nm. The particle diameters of the metal oxide particles 2 are, for example, less than or equal to 1 μm, preferably less than or equal to 100 nm, and more preferably less than or equal. to 60 nm. The cerium-containing particles 3 have a mean particle diameter greater than the mean particle diameter of the metal oxide particles 2. For example, the mean particle diameter of the cerium-containing particles 3 is greater than or equal to 0.5 μm, preferably greater than or equal to 1 μm, and more preferably greater than or equal to 2 μm. The particle diameters of the cerium-containing particles 3 are, for example, less than or equal to 30 μM, preferably less than or equal to 20 μm, and more preferably less than or equal to 10 μm.

For example, the mean particle diameter of the metal oxide particles 2 is obtained by calculating an average value of the particle diameters of the metal oxide particles 2 in an image of the metal oxide particles 2, captured at a predetermined magnification by an SEM. The same applies to the mean particle diameter of the cerium-containing particles 3. Alternatively, the mean particle diameters of the metal oxide particles 2 and the cerium-containing particles 3 may be obtained by laser diffractometry. As another alternative, the diameters of crystallites of the metal oxide particles 2 and the cerium-containing particles 3 obtained by X-ray diffractometry (XRD) may be regarded as the mean particle diameters.

The metal oxide particles 2 are fine particles of an oxide that has a spinel structure (i.e., spinel crystal structure) containing at least one of iron (Fe), manganese (Mn), and cobalt (Co) elements. For example, the metal oxide particles 2 are composed of only an oxide that contains at least one of Fe, Mn, and Co. Preferably, the metal oxide particles 2 are particles of an oxide that has a spinel structure containing Fe, Mn, and oxygen (O) (Fe_(x)Mn_(y)O₄, where x and y are positive numerical values that satisfy x+y=3) or particles of an oxide that has a spinel structure containing Co and oxygen (O) (CO₃O₄). The cerium-containing particles 3 are fine particles that contain cerium (Ce) elements. For example, the cerium-containing particles 3 are cerium dioxide (CeO₂) particles.

When the porous ceramic structure 1 contains Fe, the Fe content in the porous ceramic structure 1 is higher than or equal to 0.1 mass % and preferably higher than or equal to 1.5 mass % in terms of Fe₂O₃. The Fe content is also lower than or equal to 3.0 mass % in terms of Fe₂O₃. The Fe content in terms of Fe₂O₃ as used herein refers to the percentage of a value obtained by dividing the weight of Fe₂O₃ by the weight of the porous ceramic structure 1 when it is assumed that all Fe components contained in the porous ceramic structure 1 exist as Fe₂O₃.

When the porous ceramic structure 1 contains Mn, the Mn content in the porous ceramic structure 1 is higher than or equal to 0.1 mass % and preferably higher than or equal to 1.5 mass % in terms of Mn₂O₃. The Mn content is also lower than or equal to 3.0 mass % in terms of manganese oxide (Mn₂O₃). The Mn content in terms of Mn₂O₃ as used herein refers to the percentage of a value obtained by dividing the weight of Mn₂O₃ by the weight of the porous ceramic structure 1 when it is assumed that all Mn components contained in the porous ceramic structure 1 exist as Mn₂O₃.

When the porous ceramic structure 1 contains Co, the Co content in the porous ceramic structure 1 is as follows. When the porous ceramic structure 1 contains Co together with Fe or Mn (i.e., when the porous ceramic structure 1 contains at least one of Co, Fe, and Mn), the Co content in the porous ceramic structure 1 is higher than or equal to 0.1 mass % and preferably higher than or equal to 1.5 mass % in terms of Co₃O₄. The Co content is also lower than or equal to 3.0 mass % in terms of Co₃O₄.

Meanwhile, when the porous ceramic structure 1 contains Co without containing Fe and Mn, the Co content in the porous ceramic structure 1 is higher than or equal to 0.2 mass % and preferably higher than or equal 3.0 mass % in terms of Co₃O₄. The Co content is also lower than or equal to 6.0 mass % in terms of Co₃O₄. The Co content in terms of Co₃O₄ as used herein refers to the percentage of a value obtained by dividing the weight of Co₃O₄ by the weight of the porous ceramic structure 1 when it is assumed that all Co components contained in the porous ceramic structure 1 exist as Co₃O₄. In either of the above-described cases, most of Fe, Co, and Mn contained in the porous ceramic structure 1 exist as the metal oxide particles 2, but it is also conceivable that some of the metal oxide particles 2 are dissolved in the honeycomb structure 10.

The Ce content in the porous ceramic structure 1 is higher than or equal to 0.1 mass % and preferably higher than or equal to 1.5 mass % in terms of CeO₂. The Ce content is also lower than or equal to 10 mass % and preferably lower than or equal to 4.5 mass % in terms of CeO₂. The Ce content in terms of CeO₂ as used herein refers to the percentage of a value obtained by dividing the weight of CeO₂ by the weight of the porous ceramic structure 1 when it is assumed that all Ce components contained in the porous ceramic structure 1 exist as CeO₂. Although most of Ce contained in the porous ceramic structure 1 exists as the cerium-containing particles 3, it is also conceivable that part of Ce may be dissolved in the honeycomb structure 10.

A ratio of the sum of the Fe content in terms of Fe₂O₃, the Mn content in terms of Mn₂O₃, and the Co content in terms of Co₃O₄ to the Ce content in terms of CeO₂ (i.e., value obtained by dividing the total of the Fe content in terms of Fe₂O₃, the Mn content in terms of Mn₂O₃, and the Co content in terms of Co₃O₄ by the Ce content in terms of CeO₂) is higher than or equal to 0.8 and preferably higher than or equal to 1.0. This ratio is also lower than or equal to 9.5 and preferably lower than or equal to 4.0. In the following description, this ratio is referred to as the “Fe/Mn/Co ratio.”

The content of the metal oxide particles 2 in the porous ceramic structure 1 is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %. The content of the metal oxide particles 2 as used herein refers to the mass ratio of the metal oxide particles 2 in the crystalline phase of the porous ceramic structure 1. Preferably, the content of the metal oxide particles 2 is higher than or equal to 2.0 mass % and lower than or equal to 8.0 mass %.

The amount of the metal oxide particles 2 supported in the porous ceramic structure 1 is, for example, greater than or equal to 2.0 grams per liter (g/L), preferably greater than or equal to 3.0 g/L, and more preferably greater than or equal to 5.0 g/L. The amount of the metal oxide particles 2 supported in the porous ceramic structure 1 is also, for example, less than or equal to 50 g/L, preferably less than or equal to 45 g/L, and more preferably less than or equal to 40 g/L. The amount (g/L) of the metal oxide particles 2 indicates the amount (g) of the metal oxide particles 2 supported per unit volume (L) of the honeycomb structure 10.

FIG. 6 shows an enlarged SEM image of metal oxide particles 2 on the surface of a pore 121. As shown in FIG. 6 , the metal oxide particles 2 have forms that protrude partly from the inside of the honeycomb structure 10 into the pore 121. FIG. 7 is a sectional view of an area in the vicinity of a metal oxide particle 2 in FIG. 6 .

The metal oxide particle 2 has a fixedly attached portion 21 and a protrusion 22. The fixedly attached portion 21 is located inside the honeycomb structure 10. The language “inside the honeycomb structure 10” refers to the inside of the cordierite surrounding the pore 121 and does not refer to the inside of the pore 121 provided in the honeycomb structure 10. The fixedly attached portion 21 is a bonding portion of the metal oxide particle 2 that is bonded to the cordierite serving as the principal component of the honeycomb structure 10 and that is fixedly attached to the inside of the cordierite. In other words, the fixedly attached portion 21 is a portion of the metal oxide particle 2 that crawls into the cordierite from the surface of the pore 121 to the side opposite to the pore 121 in the honeycomb structure 10. In yet other words, the fixedly attached portion 21 is a portion of the metal oxide particle 2 whose surface is covered with the cordierite.

The protrusion 22 is a portion of the metal oxide particle 2 that protrudes from the surface of the pore 121 into the pore 121. In other words, the protrusion 22 is a portion that is exposed from the surface of the cordierite. The protrusion 22 is contiguous with the fixedly attached portion 21.

In the porous ceramic structure 1, the honeycomb structure 10 does not undergo a coating process using γ-alumina or the like (so-called wash coating). Therefore, a coating supposed to be formed by the aforementioned coating process is not formed on the surfaces of the pores 121 and, as a natural result, the metal oxide particles 2 are not fixedly attached via such a coating to the honeycomb structure 10.

Among a large number of metal oxide particles 2 contained in the porous ceramic structure 1, some metal oxide particles 2 are fixedly attached to the surfaces of pores 121 inside the pores 121 as described above, and the other metal oxide particles 2 are located in their entirety inside the honeycomb structure 10. The same applies to the cerium-containing particles 3.

Next, one example of the method of producing the porous ceramic structure 1 will be described with reference to FIG. 8 . In the production of the porous ceramic structure 1, first, a structure raw material is prepared by weighing and mixing a material for the honeycomb structure 10, a material for the cerium-containing particles 3 (e.g., CeO₂), a material for the metal oxide particles 2, and ZnO serving as an aid. The material for the metal oxide particles 2 includes at least one of Fe, Mn, and Co. For example, the material for the metal oxide particles 2 is Fe₂O₃, Mn₂O₃, or Co₃O₄. The principal component of the material for the honeycomb structure 10 is cordierite that servers as an aggregate of the honeycomb structure 10. The principal component of the material for the honeycomb structure 10 may, for example, be kaoline, talc, or alumina that is a raw material for cordierite. The material for the honeycomb structure 10 also includes other components such as a pore-forming agent and a binder. After the structure raw material is subjected to dry mixing in a kneader, water is added and the structure raw material is further kneaded into kneaded clay in the kneader (step S11).

The time required for the aforementioned dry mixing and the time required for the aforementioned kneading are, for example, 15 minutes and 30 minutes, respectively. The dry mixing time and the kneading time may be changed in various ways. The aforementioned raw material for the cerium-containing particles 3 may be salts such as cerium nitrate. The aforementioned raw material for the metal oxide particles 2 may be salts such as iron nitrate, manganese nitrate, or cobalt nitrate.

In step S11, the raw material for the cerium-containing particles 3 and the raw material for the metal oxide particles 2 are individually added to the aggregate or the like of the honeycomb structure 10, but the method of adding these raw materials may be changed in various ways. For example, the raw material for the metal oxide particles 2 may be immersed in CeO₂ and dried and fired to generate an additive, and this additive may be added to the aggregate or the like of the honeycomb structure 10. In the additive, part of the raw material for the metal oxide particles 2 is solid-dissolved in or adheres to CeO₂.

The kneaded clay prepared in step S11 is molded into a columnar shape by a vacuum kneading machine or other machines and then subjected to extrusion molding by an extruder to form a compact having a honeycomb shape (hereinafter, also referred to as a “honeycomb compact”) (step S12). This honeycomb compact includes therein a grid-like partition wall that sections the honeycomb compact into a plurality of cells serving as flow paths for a fluid. The honeycomb compact has a honeycomb diameter of 30 mm, a partition wall thickness of 12 mil (approximately 0.3 mm), a cell density of 300 cells per square inch (cps), i.e., 46.5 cells/cm², and an outer wall thickness of approximately 0.6 mm. Alternatively, in step S12, the honeycomb compact may be molded by a molding method other than extrusion molding.

Following this, the honeycomb compact prepared in step S12 is subjected to drying. There is no particular limitations on the method of drying the honeycomb compact. For example, the drying method may be hot air drying, microwave drying, dielectric drying, reduced pressure drying, vacuum drying, or freeze drying, or may be any combination of these drying methods. For example, the honeycomb compact is subjected to microwave drying so as to transpire approximately 50 to 80% of moisture, and is then subjected to hot air drying (at 60 to 100° C. for 6 to 20 hours). Preferably, the honeycomb compact is subjected to microwave drying so as to transpire approximately 70% of moisture, and is then subjected to hot air drying (at 80° C. for 12 hours). Then, the honeycomb compact is put into a degreasing furnace that is maintained at 450° C. so as to remove (i.e., degrease) organic components remaining in the honeycomb compact.

Thereafter, the honeycomb compact is subjected to a firing process (firing), so that the porous ceramic structure 1 including the honeycomb structure 10, the cerium-containing particles 3, and the metal oxide particles 2 is obtained (step S13). The porous ceramic structure 1 produced by the aforementioned production method does not contain precious metals, and therefore can be produced at low cost.

The firing processing in step S13 is, for example, conducted with a predetermined temperature profile under atmospheric pressure. FIG. 9 is a diagram illustrating a detail of the firing step in step S13. FIGS. 10 and 11 are diagrams illustrating temperature profiles during a first firing step.

In the first firing step, first, the temperature of the honeycomb compact is raised from an ordinary temperature (e.g., 20° C.) to a first temperature t₁ that is lower than or equal to a liquid-phase formation temperature t_(L) (step S21). The liquid-phase formation temperature t_(L) as used herein refers to a temperature at which liquid-phase formation of a raw material starts in the honeycomb compact that is being heated. For example, the liquid-phase formation temperature t_(L) is a temperature within the range of 1100° C. to 1170° C. FIG. 10 shows the case where the first temperature t₁ is the same as the liquid-phase formation temperature t_(L). As described above, the first temperature t₁ may be lower than the liquid-phase formation temperature t_(L). There is no particular limitations on the rate of raising the temperature to the liquid-phase formation temperature t_(L), and for example, this rate may be higher than or equal to 100° C./h and lower than or equal to 200° C./h.

Then, the temperature of the honeycomb compact is raised from the first temperature t₁ to a second temperature t₂ that is higher than the first temperature t₁ (step S22). The second temperature t₂ is higher than the liquid-phase formation temperature t_(L) and lower than a crystallization temperature t_(C) of the metal oxide particles 2. The crystallization temperature t_(C) is a temperature at which the metal oxide particles 2 start to be crystallized and deposited from the honeycomb compact that is being heated. For example, the crystallization temperature t_(C) is a temperature within the range of 1250° C. to 1350° C. The second temperature t₂ is, for example, a temperature within the range of 1170° C. to 1250° C. The rate of raising the temperature of the honeycomb compact in step S22 is higher than or equal to 100° C./h and lower than or equal to 200° C./h.

Next, the temperature of the honeycomb compact is raised from the second temperature t₂ to a third temperature t₃ higher than the second temperature t₂ (step S23). The third temperature t₃ is higher than the crystallization temperature t_(C) and lower than or equal to a maximum temperature t_(max). For example, the maximum temperature t_(max) is a temperature within the range of 1350° C. to 1500° C. FIG. 10 shows the case where the third temperature t₃ is the same as the maximum temperature t_(max). As described above, the third temperature t₃ may be lower than the maximum temperature t_(max). There are no particular limitations on the rate of raising the temperature of the honeycomb compact in step S23, and for example, this rate may be higher than or equal to 25° C./h and lower than or equal to 200° C./h and preferably higher than or equal to 100° C./h and lower than or equal to 200° C./h.

Thereafter, the temperature of the honeycomb compact is maintained at the maximum temperature t_(max) for a predetermined period of time and is then lowered to the ordinary temperature (step S24). In this way, the firing process of the honeycomb compact ends (step S13), and the porous ceramic structure 1 is produced. A time period during which the temperature is maintained at the maximum temperature t_(max) in step S24 (hereinafter, also referred to as a “maximum-temperature maintaining time period”) is, for example, longer than or equal to five hours and shorter than or equal to 50 hours. When the third temperature t₃ is lower than the maximum temperature t_(max), the temperature of the honeycomb compact is raised from the third temperature t₃ to the maximum temperature t_(max) in a time period between steps S23 and S24. There is no particular limitations on the rate of raising the temperature from the third temperature t₃ to the maximum temperature t_(max), and for example, this rate may be higher than or equal to 25° C./h and lower than or equal to 75° C./h.

In the first firing step, the rise in the temperature of the honeycomb compact may be temporarily stopped in the midst of step S23 as illustrated in FIG. 11 . Specifically, when the temperature of the honeycomb compact has been raised to the crystallization temperature t_(C) in step S23, the rise in temperature may be temporarily stopped, and a state in which the temperature of the honeycomb compact remains at the crystallization temperature t_(C) is maintained for a predetermined period of time (hereinafter, also referred to as an “intermediate maintaining time period”). The intermediate maintaining time period is, for example, longer than or equal to one hour and shorter than or equal to 100 hours, and preferably longer than or equal to five hours and shorter than or equal to 75 hours. Note that the intermediate maintaining time period may be less than one hour, and may be longer than 100 hours. The example illustrated in FIG. 10 corresponds to a temperature profile in which the intermediate maintaining time period is zero. In the example illustrated in FIG. 11 , when the intermediate maintaining time period has elapsed, the temperature of the honeycomb compact resumes rising and is raised to the third temperature t₃ (in the example illustrated in FIG. 11 , to the same temperature as the maximum temperature t_(max)).

Next, a second example of the firing step in step S13 will be described with reference to FIGS. 12 and 13 . A detailed procedure of the firing step is similar to that illustrated in FIG. 9 . FIGS. 12 and 13 are diagrams illustrating temperature profiles during a second firing step.

In the second firing step, as in the first firing step described above, the temperature of the honeycomb compact is raised from an ordinary temperature to a first temperature t₁ that is lower than or equal to a liquid-phase formation temperature t_(L) (step S21 in FIG. 9 ). The liquid-phase formation temperature t_(L) is, for example, a temperature within the range of 1100° C. to 1170° C. FIG. 12 shows the case where the first temperature t₁ is the same as the liquid-phase formation temperature t_(L). As described above, the first temperature t₁ may be lower than the liquid-phase formation temperature t_(L). There is no particular limitations on the rate of raising the temperature to the liquid-phase formation temperature t_(L), and for example, this rate may be higher than or equal to 50° C./h and lower than or equal to 200° C./h. In the example illustrated in FIG. 12 , the rate of temperature rise drops at the boundary of around 900° C.

Then, the temperature of the honeycomb compact is raised from the first temperature t₁ to a second temperature t₂ that is higher than the first temperature t₁ (step S22). The second temperature t₂ is higher than the liquid-phase formation temperature t_(L) and lower than the crystallization temperature t_(C) of the metal oxide particles 2. The crystallization temperature t_(C) is, for example, a temperature within the range of 1250° C. to 1350° C. For example, the second temperature t₂ is within the range of 1170° C. to 1250° C. Unlike in the first firing step, the rate of raising the temperature of the honeycomb compact in step S22 is higher than or equal to 1° C./h and lower than or equal to 10° C./h.

Next, the temperature of the honeycomb compact is raised from the second temperature t₂ to a third temperature t₃ that is higher than the second temperature t₂ (step S23). The third temperature t₃ is higher than the crystallization temperature t_(C) and lower than or equal to a maximum temperature t_(max). The maximum temperature t_(max) is, for example, a temperature within the range of 1350° C. to 1500° C. FIG. 12 shows the case where the third temperature t₃ is the same as the maximum temperature t_(max). As described above, the third temperature t₃ may be lower than the maximum temperature t_(max). There are no particular limitations on the rate of raising the temperature of the honeycomb compact in step S23, and for example, this rate may be higher than or equal to 25° C./h and lower than or equal to 200° C./h.

Thereafter, the temperature of the honeycomb compact is maintained at the maximum temperature t_(max) for a predetermined period of time and then lowered to the ordinary temperature (step S24). In this way, the firing process of the honeycomb compact (step S13) ends, and the porous ceramic structure 1 is produced. A time period during which the temperature is maintained at the maximum temperature t_(max) in step S24 (i.e., a “maximum-temperature maintaining time period”) is, for example, longer than or equal to five hours and shorter than or equal to 50 hours. When the third temperature t₃ is lower than the maximum temperature t_(max), the temperature of the honeycomb compact is raised from the third temperature t₃ to the maximum temperature t_(max) in a time period between steps S13 and 14. There are no particular limitations on the rate of raising the temperature from the third temperature t₃ to the maximum temperature t_(max), and for example, this rate may be higher than or equal to 25° C./h and lower than or equal to 75° C./h.

In the second firing step, the rise in the temperature of the honeycomb compact may be temporarily stopped in the midst of step S23 as illustrated in FIG. 13 . Specifically, when the temperature of the honeycomb compact has been raised to the crystallization temperature t_(C) in step S23, the rise in temperature is temporarily stopped and a state in which the temperature of the honeycomb compact remains at the crystallization temperature t_(C) is maintained for a predetermined period of time (i.e., an “intermediate maintaining time period”). The intermediate maintaining time period is, for example, longer than or equal to one hour and shorter than or equal to 100 hours, and preferably longer than or equal to five hours and shorter than or equal to 75 hours. Note that the intermediate maintaining time period may be shorter than one hour, and may be longer than 100 hours. The example illustrated in FIG. 12 corresponds to a temperature profile in which the intermediate maintaining time period is zero. In the example illustrated in FIG. 13 , when the intermediate maintaining time period has elapsed, the temperature of the honeycomb compact resumes rising and is raised to the third temperature t₃ (in the example illustrated in FIG. 13 , to the same temperature as the maximum temperature t_(max)).

Next, a third example of the firing step in step S13 will be described with reference to FIG. 14 . A detailed procedure in the firing step is similar to that in FIG. 9 . FIG. 14 is a diagram illustrating a temperature profile during a third firing step.

In the third firing step, as in the first and second firing steps described above, the temperature of the honeycomb compact is raised from an ordinary temperature to a first temperature t₁ that is lower than or equal to a liquid-phase formation temperature t_(L) (step S21 in FIG. 9 ). The liquid-phase formation temperature t_(L), is, for example, a temperature within the range of 1100° C. to 1170° C. FIG. 14 shows the case where the first temperature t₁ is the same as the liquid-phase formation temperature t_(L). As described above, the first temperature t₁ may be lower than the liquid-phase formation temperature t_(L). There are no particular limitations on the rate of raising the temperature to the liquid-phase formation temperature t_(L), and for example, this rate may be higher than or equal to 50° C./h and lower than or equal to 200° C./h. In the example illustrated in FIG. 14 , the rate of temperature rise drops at a boundary of around 900° C.

Than, the temperature of the honeycomb compact is raised from the first temperature t₁ to a second temperature t₂ that is higher than the first temperature t₁ (step S22). The second temperature t₂ is higher than the liquid-phase formation temperature t_(L) and lower than the crystallization temperature t_(C) of the metal oxide particles 2. The crystallization temperature t_(C) is, for example, a temperature within the range of 1250° C. to 1350° C. For example, the second temperature t₂ is within the range of 1170° C. to 1250° C. Unlike in the first and second firing steps, there are no particular limitations on the rate of raising the temperature of the honeycomb compact in step S22. The rate of temperature rise is, for example, higher than or equal to 10° C./h and lower than or equal to 100° C./h.

Next, the temperature of the honeycomb compact is raised from the second temperature t₂ to a third temperature t₃ that is higher than the second temperature t₂ (step S23). The third temperature t₃ is higher than the crystallization temperature t_(C) and lower than or equal to the maximum temperature t_(max). The maximum temperature t_(max) is, for example, a temperature within the range of 1350° C. to 1500° C. FIG. 14 shows the case where the third temperature t₃ is the same as the maximum temperature t_(max). As described above, the third temperature t₃ may be lower than the maximum temperature t_(max). There are no particular limitations on the rate of raising the temperature of the honeycomb compact in step S23, and for example, this rate may be higher than or equal to 10° C./h and lower than or equal to 100° C./h.

In the third firing step, the rise in the temperature of the honeycomb compact is temporarily stopped in the midst of step S23. Specifically, when the temperature of the honeycomb compact has raised to the crystallization temperature t_(C) in step S23, the rise in temperature is temporarily stopped, and a state in which the temperature of the honeycomb compact remains at the crystallization temperature t_(C) is maintained for a predetermined period of time (i.e., an “intermediate maintaining time period”). For example, the intermediate maintaining time period may be longer than or equal to one hour and shorter than or equal to 100 hours and preferably longer than or equal to five hours and shorter than or equal to 75 hours. Note that the intermediate maintaining time period may be less than one hour, and may be longer than 100 hours. When the intermediate maintaining time period has elapsed, the temperature of the honeycomb compact resumes rising and is raised to the third temperature t₃ (in the example illustrated in FIG. 14 , to the same temperature as the maximum temperature t_(max)).

Thereafter, the temperature of the honeycomb compact is maintained at the maximum temperature t_(max) for a predetermined period of time, and is then lowered to the ordinary temperature (step S24). In this way, the firing process of the honeycomb compact (step S13) ends, and the porous ceramic structure 1 is produced. A time period during which the temperature is maintained at the maximum temperature t_(max) in step S24 (i.e., maximum-temperature maintaining time period) is, for example, longer than or equal to five hours and shorter than or equal to 50 hours. When the third temperature t₃ is lower than the maximum temperature t_(max), the temperature of the honeycomb compact is raised from the third temperature t₃ to the maximum temperature t_(max) in a time period between steps S23 and S24. There are no particular limitations on the rate of raising the temperature from the third temperature t₃ to the maximum temperature t_(max), and for example, this rate may be higher than or equal to 25° C./h and lower than or equal to 75° C./h.

Next, pressure loss and catalytic performance in the porous ceramic structures 1 according to Examples 1 to 10 will be described with reference to Tables 1 to 4. Comparative Example 1 will also be described in the same manner. Among Examples 1 to 10, Examples 1 to 6 and Examples 7 to 10 differ in material composition and crystalline phase composition as will be described later (see Tables 2 and 3). Note that the material composition shown in Table 2 indicates the composition of materials used to produce the porous ceramic structure 1 (i.e., prepared composition). The crystalline phase composition shown in Table 3 is the crystalline phase composition in the produced porous ceramic structure 1.

TABLE 1 Intermediate Temperature-Rise Temperature-Rise Maintaining Maximum-Temperature Rate C2 Rate C3 Time Period Maintaining Time Period (° C./h) (º C./h) (h) (h) Example 1 12 60 10 7 Example 2 12 60 50 7 Example 3 3 60 0 7 Example 4 180 60 0 7 Example 5 200 200 0 7 Example 6 180 60 10 7 Example 7 12 60 10 7 Example 8 3 60 0 7 Example 9 180 60 0 7 Example 10 200 200 0 7 Comparative 12 60 0 7 Example 1

TABLE 2 Material Composition (mass %) Fe/Mn/Co MgO Al₂O₃ SiO₂ Fe₂O₃ Mn₂O₃ Co₃O₄ CeO₂ Total Ratio Example 1 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 2 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 3 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 4 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 5 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 6 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 7 12.7 31.0 46.3 0.0 0.0 5.5 4.5 100.0 1.2 Example 8 12.7 31.0 46.3 0.0 0.0 5.5 4.5 100.0 1.2 Example 9 12.7 31.0 46.3 0.0 0.0 5.5 4.5 100.0 1.2 Example 10 12.7 31.0 46.3 0.0 0.0 5.5 4.5 100.0 1.2 Comparative 12.7 31.0 46.3 2.6 2.4 0.0 5.0 100.0 1.0 Example 1

TABLE 3 Crystalline Phase Composition (mass %) Cordierite Fe_(x)Mn_(y)O₄ Co₃O₄ CeO₂ Others Total Example 1 87.5 2.5 0.0 2.2 7.8 100.0 Example 2 88.2 2.0 0.0 1.8 8.0 100.0 Example 3 85.3 3.8 0.0 3.6 7.3 100.0 Example 4 83.1 5.2 0.0 4.8 6.9 100.0 Example 5 81.8 6.0 0.0 5.8 6.4 100.0 Example 6 83.0 5.4 0.0 4.6 7.0 100.0 Example 7 87.3 0.0 2.6 2.4 7.7 100.0 Example 8 85.2 0.0 3.9 3.6 7.3 100.0 Example 9 82.1 0.0 5.2 4.9 7.8 100.0 Example 10 82.2 0.0 5.8 5.5 6.5 100.0 Comparative 87.0 1.8 0.0 3.0 8.2 100.0 Example 1

TABLE 4 Open NO NO Oxidation Mean Particle Diameter (nm) Porosity Adsorption Temperature Fe_(x)Mn_(y)O₄ Co₃O₄ CeO₂ (%) (μmol/g) (° C.) Example 1 20 — 21 55 0.01 520 Example 2 22 — 23 52 0.01 520 Example 3 18 — 19 57 0.02 510 Example 4 20 — 22 62 0.04 490 Example 5 18 — 21 62 0.05 490 Example 6 19 — 22 60 0.04 490 Example 7 — 19 20 55 0.01 520 Example 8 — 19 21 57 0.02 510 Example 9 — 20 23 62 0.02 500 Example 10 — 15 20 62 0.02 500 Comparative 50 — 55 58 0.00 540 Example 1

The porous ceramic structures 1 according to Examples 1, 2, and 7 were fired in the aforementioned third firing step. The porous ceramic structures 1 according to Examples 3 and 8 were fired in the aforementioned second firing step. The porous ceramic structures 1 according to Examples 4 to 6, 9, and 10 were fired in the aforementioned first firing step. Temperature-rise rate C2 in the table indicates the rate of temperature rise (° C./h) in step S22 described above (i.e., the rate of temperature rise per hour). Temperature-Rise Rate C3 indicates the rate of temperature rise (° C./h) in step S23 described above. Intermediate Maintaining Time Period (h) indicates the time period during which the temperature is maintained at the crystallization temperature t_(C) in step S23 described above. Maximum-Temperature Maintaining Time Period (h) indicates the time period during which the temperature is maintained at the maximum temperature t_(max) in step S24 described above.

In Examples 1, 2, and 7, the intermediate maintaining time period in the third firing step was changed. In Examples 1, 2, and 7, Temperature-Rise Rate C2 was 12° C./h and out of both the range from 1° C./h to 10° C./h and the range from 100° C./h to 200° C./h. The temperature profiles according to Examples 1, 2, and 7 were the type illustrated in FIG. 14 . In Examples 3 and 8, Temperature-Rise Rate C2 during the second firing step was 3° C./h (i.e., higher than or equal to 1° C./h and lower than or equal to 10° C./h). Note that the intermediate maintaining time period was not set in Examples 3 and 8. The temperature profiles according to Examples 3 and 8 were the type illustrated in FIG. 12 .

In Examples 4 to 6, 9, and 10, Temperature-Rise Rate C2 during the first firing step was in the range of 180° C./h to 200° C./h (i.e., higher than or equal to 100° C. and lower than equal to 200° C./h). In Examples 4, 5, 9, and 10, Temperature-Rise Rate C2 was higher than or equal to 100° C./h and lower than or equal to 200° C./h, and Temperature-Rise Rate C3 was changed. Note that the intermediate maintaining time period was not set in Examples 4, 5, 9, and 10. The temperature profiles according to Examples 4, 5, 9, and 10 were the type illustrated in FIG. 10 . In Example 6, Temperature-Rise Rate C2 was higher than or equal to 100° C./h and lower than or equal to 200° C./h, and the intermediate maintaining time period was set to 10 hours. The temperature profile according to Example 6 was the type illustrated in FIG. 11 .

The Fe/Mn/Co ratio in the table was obtained by dividing the total of the Fe content (mass %) in terms of Fe₂O₃, the Mn content (mass %) in terms of Mn₂O₃, and the Co content (mass %) in terms of Co₃O₄ by the CeO₂ content (mass %) in the material composition of the porous ceramic structure 1.

The crystalline phase composition in the porous ceramic structure 1 (i.e., mass ratio of components of the crystalline phase) was identified and quantitated as follows. Measurements of the crystalline phase of each particle were conducted on a produced sample, using an X-ray diffractometer (rotary anti-cathode X-ray diffractometer: RINT 2500 manufactured by Rigaku Corporation). Here, conditions for the X-ray diffractometry were a CuKα-ray source, 50 kV, 300 mA, and 2θ of 10° to 60°, and resultant X-ray diffraction data was analyzed using commercial X-ray data analysis software. In the crystalline phase, Fe_(x)Mn_(y)O₄ was a spinel oxide composed of Fe and Mn. In Fe_(x)Mn_(y)O₄, x and y were positive numerical values that satisfy x+y=3. A mean particle diameter of Fe_(x)Mn_(y)O₄, where x and y were positive numerical values that satisfy x+y=3, was assumed to be the diameter of crystallites. The diameter of crystallites was calculated, based on the data obtained by the aforementioned X-ray diffractometry using the X-ray diffractometer, by applying this data to the Scherrer equation (τ=Kλ/(βcosθ). Here, τ was the average size of crystallites, K was the form factor (factor forming a relationship between the sizes of crystallites contained in a solid and the peak width of a diffraction pattern), λ was the X-ray wavelength, β was the full width at peak (in units of radians), and θ was the Bragg angle. In the crystalline phase, Co₃O₄ was a spinel oxide of Co. A mean particle diameter of Co₃O₄ was obtained in the same manner as the mean particle diameter of Fe_(x)Mn_(y)O₄. A mean particle diameter of CeO₂ was also obtained in the same manner. Quantifications of Fe_(x)Mn_(y)O₄, Co₃O₄, and CeO₂ were performed by Rietveld analysis using the obtained X-ray diffraction data. The open porosity of the porous ceramic structure 1 was measured by the Archimedes method using deionized water as a medium. As described above, the porous ceramic structure 1 had lower pressure loss with increasing open porosity.

The NO adsorption in the porous ceramic structure 1 was obtained as follows. First, an adsorption test was conducted by supplying an NO-containing introduction gas to a sample equivalent to the porous ceramic structure 1. The introduction gas was a mixed gas containing 200 volume ppm of NO and 10 volume % of oxygen (O₂) and using helium (He) as a balance gas. The adsorption test was conducted at 250° C. for 60 minutes. After the adsorption test ended, the sample was subjected to temperature-programmed desorption in He, and a resultant derived gas was partly sampled. The sampled derived gas was then analyzed using a mass spectrometer (GSD 320 manufactured by Pfeiffer Vacuum) so as to obtain the amount of NO adsorbed in the sample. In general, there is tendency that the porous ceramic structure 1 exhibits higher catalytic performance with increasing NO adsorption.

The NO oxidation temperature of the porous ceramic structure 1 was obtained as follows. First, the relationship between the temperature and the NO₂ conversion rate in the porous ceramic structure 1 was obtained. The NO₂ conversion rate was the ratio of conversion from NO to NO₂ in an NO-containing derived gas that had been supplied to the porous ceramic structure 1 at a space velocity (SV) of 24400 h⁻¹ and passed through the porous ceramic structure 1. An initial gas contained 100 ppm of NO, 1500 ppm of CO, 5% of CO₂, 450 ppm of propane (C₃H₆), and 2% of H₂O. Analysis of the derived gas was conducted by Fourier transform infrared spectroscopy (FT-IR). The NO₂ conversion rate was approximately 0% at low temperatures, gradually increased to a maximum value with increasing temperature, and then gradually decreased. The porous ceramic structure 1 exhibited higher catalytic performance as the NO₂ conversion rate increased. When the relationship between the NO₂ conversion rate and the temperature was obtained, the temperature was increased from the low temperature side in accordance with the above relationship, and a temperature at which the NO₂ conversion rate became one half of the maximum value was obtained as the NO oxidation temperature. The porous ceramic structure 1 exhibited higher catalytic performance as the NO oxidation temperature decreased.

In Examples 1 to 6, the Fe content is 2.6 mass % (i.e., higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass %) in terms of Fe₂O₃, the Mn content is 2.4 mass % (i.e., higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass %) in terms of Mn₂O₃, and the Ce content is 5.0 mass % (i.e., higher than or equal to 0.1 mass % and lower than or equal to 10 mass %) in terms of CeO₂. The porous ceramic structures 1 according to Examples 1 to 6 substantially do not contain Co, and therefore the Co content therein is 0.0 mass % in terms of Co₃O₄. The Fe/Mn/Co ratio (i.e., the ratio of the sum of the Fe content in terms of Fe₂O₃, the Mn content in terms of Mn₂O₃, and the Co content in terms of Co₃O₄ to the Ce content in terms of CeO₂) is 1.0 (i.e., higher than or equal to 0.8 and lower than or equal to 9.5).

The porous ceramic structures 1 according to Examples 7 to 10 substantially do not contain Fe and Mn, and therefore the Fe content is 0.0 mass % in terms of Fe₂O₃ and the Mn content is 0.0 mass % in terms of Mn₂O₃. The Co content is 5.5 mass % (i.e., higher than or equal to 3.0 mass % and lower than or equal to 6.0 mass %) in terms of Co₃O₄, and the Ce content is 4.5 mass % (i.e., higher than or equal to 1.5 mass % and lower than or equal to 4.5 mass %) in terms of CeO₂. The Fe/Mn/Co ratio is 1.2 (i.e., higher than or equal to 1.0 and lower than or equal to 4.0). In this case, the Fe/Mn/Co ratio is substantially the ratio of the Co content in terms of Co₃O₄ to the Ce content in terms of CeO₂.

FIG. 15 shows an SEM image of the surface of the porous ceramic structure 1 according to Example 3. FIG. 16 shows an enlarged SEM image of the surface of a pore 121 in the porous ceramic structure 1 according to Example 3. FIG. 17 shows an SEM image of the surface of the porous ceramic structure 1 according to Example 4. FIG. 18 shows an enlarged SEM image of the surface of a pore 121 in the porous ceramic structure 1 according to Example 4. In FIGS. 15 and 17 , black portions correspond to pores 121, gray portions correspond to the honeycomb structure 10, and white portions correspond to the metal oxide particles 2 and the cerium-containing particles 3. In FIGS. 15 to 18 , the metal oxide particles 2 and the cerium-containing particles 3 exist on the surfaces of the pores 121 in the porous ceramic structure 1.

In Examples 1 to 10, the content of the metal oxide particles 2 is in the range of 2.0 mass % to 6.0 mass % (i.e., higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %). As described above, the content of the metal oxide particles 2 as used herein refers to the mass ratio of the metal oxide particles 2 in the crystalline phase and the total of the mass ratios of Fe_(x)Mn_(y)O₄ and Co₃O₄ in the crystalline phase in Table 3. In Examples 1 to 10, the mean particle diameter of the metal oxide particles 2 is in the range of 15 nm to 22 nm (i.e., greater than or equal to 10 nm and less than or equal to 1 μm).

In Examples 1 to 10, the open porosity is in the range of 52% to 62% and relatively high, so that pressure loss in the porous ceramic structure 1 is kept low. In Examples 1 to 10, the NO absorption is in the range of 0.01 (μmol/g) to 0.05 (mol/g) and the NO oxidation temperature is in the range of 490° C. to 520° C. and low. This indicates that the porous ceramic structures 1 exhibit high catalytic performance. Besides, in Example 5 with Temperature-Rise Rate C3 of 200° C./h (i.e., higher than or equal to 100° C./h and lower than or equal to 200° C./h), the NO absorption is 0.05 (mol/g) and the highest, which is higher than the NO absorption (0.04 mol/g) in Example 4 with Temperature-Rise Rate C3 of 60° C./h.

On the other hand, in Comparative Example 1, the raw-material composition is the same as the raw-material compositions in Examples 1 to 6, but Temperature-Rise Rate C2 is 12° C./h. That is, Temperature-Rise Rate C2 in Comparative Example 1 is out of both the range from 1° C./h to 10° C./h (Examples 3 and 8) and the range from 100° C./h to 200° C./h (Examples 4 to 6, 9, and 10). Unlike in Examples 1, 2, 6, and 7, in Comparative Example 1, the intermediate maintaining time period is not set in the temperature profile. Thus, the mass ratio of the metal oxide particles 2 in the crystalline phase is 1.8 mass % and lower than the mass ratios in Examples 1 to 6 (from 2.0 mass % to 6.0 mass %). Besides, the mean particle diameter of the metal oxide particles 2 is 50 nm and higher than the mean particle diameters in Examples 1 to 6 (from 18 nm to 22 nm). Accordingly, in Comparative Example 1, the NO absorption is 0.00 (μmol/g) and the NO oxidation temperature is 540° C. and high.

As described above, the porous ceramic structure 1 includes the structure body (i.e., honeycomb structure 10), the cerium-containing particles 3, and the metal oxide particles 2. The honeycomb structure 10 is a porous member composed primarily of cordierite. The cerium-containing particles 3 are fixedly attached to the honeycomb structure 10. The metal oxide particles 2 are particles of an oxide that has a spinel structure containing at least one of Fe, Mn, and Co. The metal oxide particles 2 are fixedly attached to the insides of gas cavities (i.e., pores 121) in the honeycomb structure 10. The metal oxide particles 2 each include the fixedly attached portion 21 and the protrusion 22. The fixedly attached portion 21 is located inside the honeycomb structure 10. The protrusion 22 is contiguous with the fixedly attached portion 21 and protrudes into a pore 121.

When the porous ceramic structure 1 contains Fe, the Fe content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, and when the porous ceramic structure 1 contains Mn, the Mn content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃. When the porous ceramic structure 1 contains Co together with Fe or Mn, the Co content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, and when the porous ceramic structure 1 contains only Co without containing Fe and Mn, the Co content is higher than or equal to 0.2 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄. The Ce content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO₂. The ratio of the sum of the Fe content in terms of Fe₂O₃, the Mn content in terms of Mn₂O₃, and the Co content in terms of Co₃O₄ to the Ce content in terms of CeO₂ (i.e., Fe/Mn/Co ratio) is higher than or equal to 0.8 ad lower than or equal to 9.5. The content of the metal oxide particles 2 is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.

Accordingly, as shown in Examples 1 to 10, it is possible to reduce pressure loss in the porous ceramic structure 1 and increase the NO adsorption in the porous ceramic structure 1. It is also possible to increase the NO₂ conversion rate and reduce the NO combustion temperature in the porous ceramic structure 1. Moreover, it is possible to increase the rate of conversion from CO to CO₂ and the rate of conversion from CH to CO₂ and H₂O. In other words, employing the above-described configuration of the porous ceramic structure 1 makes it possible to provide the porous ceramic structure 1 with low pressure loss and high catalytic performance.

In the porous ceramic structure 1, the Fe/Mn/Co ratio is higher than or equal to 0.8 and lower than or equal to 9.5 as described above. It is thus possible, in the production of the porous ceramic structure 1, to favorably generate the metal oxide particles 2 of an oxide having a spinel structure. Since the content of the metal oxide particles 2 is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass % as described above, the porous ceramic structure 1 can favorably achieve both a reduction in pressure loss and an improvement in catalytic performance.

The porous ceramic structure 1 with low pressure loss and high catalytic performance as described above is in particular suitable for use in a diesel particulate filter that collects particulate matter in an exhaust gas emitted from a diesel engine.

As described above, when the porous ceramic structure 1 contains Fe, the Fe content is preferably higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, and when the porous ceramic structure 1 contains Mn, the Mn content is preferably higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃. When the porous ceramic structure 1 contains Co together with Fe or Mn, the Co content is preferably higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, and when the porous ceramic structure 1 contains Co without containing Fe and Mn, the Co content is preferably higher than or equal to 3.0 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄. Moreover, the Ce content is preferably higher than or equal to 1.5 mass % and lower than or equal to 4.5 mass % in terms of CeO₂. The ratio of the sum of the Fe content in terms of Fe₂O₃, the Mn content in terms of Mn₂O₃, and the Co content in terms of Co₃O₄ to the Ce content in terms of CeO₂ (i.e., Fe/Mn/Co ratio) is preferably higher than or equal to 1.0 and lower than or equal to 4.0. This allows the porous ceramic structure 1 to exhibit higher catalytic performance.

As described above, the metal oxide particles 2 are more preferably particles of an oxide that has a spinel structure containing Fe, Mn, and oxygen or particles of an oxide that has a spinel structure containing Co and oxygen. This allows the porous ceramic structure 1 to exhibit higher catalytic performance.

As described above, the mean particle diameter of the metal oxide particles 2 is preferably greater than or equal to 10 nm and less than or equal to 1 μm. This allows the porous ceramic structure 1 to favorably achieve both a reduction in pressure loss and an improvement in catalytic performance.

The aforementioned method of producing the porous ceramic structure 1 includes the step of preparing kneaded clay by kneading raw materials (step S11), the step of obtaining a compact by molding the kneaded clay (step S12), and the step of firing the compact (step S13). The above raw materials include cordierite, Ce, and at least one of Fe, Mn, and Co. Step S13 forms the porous ceramic structure 1 that includes the porous structure body (i.e., honeycomb structure 10) composed primarily of cordierite, the cerium-containing particles 3 fixedly attached to the honeycomb structure 10, and the metal oxide particles 2, i.e., particles of an oxide that has a spinel structure containing at least one of Fe, Mn, and Co and that are fixedly attached to the insides of gas cavities (i.e., pores 121) in the honeycomb structure 10.

Step S13 includes the step of raising the temperature of the aforementioned compact to the first temperature t₁ lower than or equal to the liquid-phase formation temperature t_(L) (step S21), the step of raising the temperature of the compact from the first temperature t₁ to the second temperature t₂ higher than the liquid-phase formation temperature t_(L) and lower than the crystallization temperature t_(C) of the metal oxide particles 2 (step S22), the step of raising the temperature of the compact from the second temperature t₂ to the third temperature t₃ higher than the crystallization temperature t_(C) and lower than or equal to the maximum temperature t_(max) (step S23), and the step of maintaining the temperature of the compact at the maximum temperature t_(max) for a predetermined period of time (i.e., maximum-temperature maintaining time period) (step S24). In the aforementioned first firing step, Temperature-Rise Rate C2 in step S22 is higher than or equal to 100° C./h and lower than or equal to 200° C./h. This production method enables readily producing the porous ceramic structure 1 with low pressure loss and high catalytic performance as shown in Examples 4 to 6, 9, and 10.

As described above, Temperature-Rise Rate C3 in step S23 is also preferably higher than or equal to 100° C./h and lower than or equal to 200° C./h. This allows the porous ceramic structure 1 to exhibit high catalytic performance.

Preferably, in step S23, the temperature of the aforementioned compact is maintained at the crystallization temperature t_(C) for a predetermined period of time (i.e., intermediate maintaining time period). This allows the porous ceramic structure 1 to exhibit high catalytic performance.

Step S13 described above includes steps S21 to S24, and in the aforementioned second firing step, Temperature-Rise Rate C2 in step S22 is higher than or equal to 1° C./h and lower than or equal to 10° C./h. This production method enables readily producing the porous ceramic structure 1 with low pressure loss and high catalytic performance as shown in Examples 3 and 8.

Preferably, in step S23, the temperature of the aforementioned component is maintained at the crystallization temperature t_(C) for a predetermined period of time (i.e., intermediate maintaining time period). This allows the porous ceramic structure 1 to exhibit higher catalytic performance.

Step S13 described above includes step S21 to S24, and in the aforementioned third firing step, the temperature of the aforementioned compact is maintained at the crystallization temperature t_(C) for a predetermined period of time (i.e., intermediate maintaining time period). This production method enables readily producing the porous ceramic structure 1 with low pressure loss and high catalytic performance as shown in Examples 1, 2, and 7.

The porous ceramic structure 1 and the method of producing the porous ceramic structure 1 described above may be modified in various ways.

For example, the mean particle diameter of the metal oxide particles 2 may be less than 10 nm, and may be greater than 1 μm.

The metal oxide particles 2 may be particles of an oxide that has a spinel structure containing only one of Fe and Mn. The metal oxide particles 2 may also be particles of an oxide that has a spinel structure containing Fe and Co but not containing Mn, or may be particles of an oxide that has a spinel structure containing Mn and Co but not containing Fe. The metal oxide particles 2 may also be particles of an oxide that has a spinel structure containing all of Fe, Mn, and Co. The metal oxide particles 2 may contain a metal other than Fe, Mn, and Co.

In the porous ceramic structure 1, particles other than the cerium-containing particles 3 and the metal oxide particles 2 may be fixedly attached to the honeycomb structure 10 (i.e., structure body).

In the porous ceramic structure 1, the shape of the aforementioned structure body is not limited to a honeycomb shape, and may be any of various shapes (e.g., generally cylindrical shape) other than the honeycomb shape.

The method of producing the porous ceramic structure 1 is not limited to the method described above, and may be modified in various ways.

The porous ceramic structure 1 may be used in applications other than for use in diesel particulate filters.

The configurations of the preferred embodiments and variations described above may be appropriately combined as long as there are no mutual inconsistencies.

While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore to be understood that numerous modifications and variations can be devised without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

The present invention is applicable to filters that collect particulate matter, e.g., diesel particulate filters that collect particulate matter in exhaust gases emitted from diesel engines.

REFERENCE SIGNS LIST

-   -   1 Porous ceramic structure     -   2 Metal oxide particles     -   3 Cerium-containing particles     -   10 Honeycomb structure     -   21 Fixedly attached portion     -   22 Protrusion     -   81 DPF     -   83 CSF     -   S11 to S13, S21 to S24 Step 

The invention claimed is:
 1. A porous ceramic structure comprising: a porous structure body composed primarily of cordierite; cerium-containing particles fixedly attached to said structure body; and metal oxide particles each fixedly attached to an inside of a pore in said structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt, wherein said metal oxide particles include: a fixedly attached portion located inside said structure body; and a protrusion contiguous with said fixedly attached portion and protruding into said pore, when said porous ceramic structure contains iron, an iron content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, when said porous ceramic structure contains manganese, a manganese content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃, when said porous ceramic structure contains cobalt together with iron or manganese, a cobalt content is higher than or equal to 0.1 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, when said porous ceramic structure contains cobalt without containing iron and manganese, the cobalt content is higher than or equal to 0.2 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄, a cerium content is higher than or equal to 0.1 mass % and lower than or equal to 10 mass % in terms of CeO₂, a ratio of a sum of the iron content in terms of Fe₂O₃, the manganese content in terms of Mn₂O₃, and the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 0.8 and lower than or equal to 9.5, and a content of said metal oxide particles is higher than or equal to 0.3 mass % and lower than or equal to 8.0 mass %.
 2. The porous ceramic structure according to claim 1, wherein when said porous ceramic structure contains iron, the iron content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Fe₂O₃, when said porous ceramic structure contains manganese, the manganese content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Mn₂O₃, when said porous ceramic structure contains cobalt together with iron or manganese, the cobalt content is higher than or equal to 1.5 mass % and lower than or equal to 3.0 mass % in terms of Co₃O₄, when said porous ceramic structure contains cobalt without containing iron and manganese, the cobalt content is higher than or equal to 3.0 mass % and lower than or equal to 6.0 mass % in terms of Co₃O₄, the cerium content is higher than or equal to 1.5 mass % and lower than or equal to 4.5 mass % in terms of CeO₂, and the ratio of the sum of the iron content in terms of Fe₂O₃, the manganese content in terms of Mn₂O₃, and the cobalt content in terms of Co₃O₄ to the cerium content in terms of CeO₂ is higher than or equal to 1.0 and lower than or equal to 4.0.
 3. The porous ceramic structure according to claim 1, wherein said metal oxide particles are particles of an oxide that has a spinel structure containing iron, manganese, and oxygen, or particles of an oxide that has a spinel structure containing cobalt and oxygen.
 4. The porous ceramic structure according to claim 1, wherein said metal oxide particles have a mean particle diameter greater than or equal to 10 nm and less than or equal to 1 μm.
 5. The porous ceramic structure according to claim 1, for use in a diesel particulate filter that collects particulate matter in an exhaust gas emitted from a diesel engine.
 6. A method of producing a porous ceramic structure, comprising: a) preparing kneaded clay by kneading a raw material; b) obtaining a compact by molding said kneaded clay; and c) firing said compact, wherein said raw material includes: cordierite; cerium; and at least one of iron, manganese, and cobalt, said operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to said structure body, and metal oxide particles each fixedly attached to an inside of a pore in said structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt, said operation c) includes: c1) raising a temperature of said compact to a first temperature that is lower than or equal to a liquid-phase formation temperature; c2) raising the temperature of said compact from said first temperature to a second temperature that is higher than said liquid-phase formation temperature and lower than a crystallization temperature of said metal oxide particles; c3) raising the temperature of said compact from said second temperature to a third temperature that is higher than said crystallization temperature and lower than or equal to a maximum temperature; and c4) maintaining the temperature of said compact at said maximum temperature for a predetermined period of time, and a rate of raising the temperature in said operation c2) is higher than or equal to 100° C./h and lower than or equal to 200° C./h.
 7. The method of producing a porous ceramic structure according to claim 6, wherein a rate of raising the temperature in said operation c3) is higher than or equal to 100° C./h and lower than or equal to 200° C./h.
 8. The method of producing a porous ceramic structure according to claim 6, wherein in said operation c3), the temperature of said compact is maintained at said crystallization temperature for a predetermined period of time.
 9. A method of producing a porous ceramic structure, comprising: a) preparing kneaded clay by kneading a raw material; b) obtaining a compact by molding said kneaded clay; and c) firing said compact, wherein said raw material includes: cordierite; cerium; and at least one of iron, manganese, and cobalt, said operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to said structure body, and metal oxide particles each fixedly attached to an inside of a pore in said structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt, said operation c) includes: c1) raising a temperature of said compact to a first temperature that is lower than or equal to a liquid-phase formation temperature; c2) raising the temperature of said compact from said first temperature to a second temperature that is higher than said liquid-phase formation temperature and lower than a crystallization temperature of said metal oxide particles; c3) raising the temperature of said compact from said second temperature to a third temperature that is higher than said crystallization temperature and lower than or equal to a maximum temperature; and c4) maintaining the temperature of said compact at said maximum temperature for a predetermined period of time, and a rate of raising the temperature in said operation c2) is higher than or equal to 1° C./h and lower than or equal to 10° C./h.
 10. The method of producing a porous ceramic structure according to claim 9, wherein in said operation c3), the temperature of said compact is maintained at said crystallization temperature for a predetermined period of time.
 11. A method of producing a porous ceramic structure, comprising: a) preparing kneaded clay by kneading a raw material; b) obtaining a compact by molding said kneaded clay; and c) firing said compact, wherein said raw material includes: cordierite; cerium; and at least one of iron, manganese, and cobalt, said operation c) forms a porous ceramic structure that includes a porous structure body composed primarily of cordierite, cerium-containing particles fixedly attached to said structure body, and metal oxide particles each fixedly attached to an inside of a pore in said structure body, the metal oxide particles being particles of an oxide that has a spinel structure containing at least one of iron, manganese, and cobalt, said operation c) includes: c1) raising a temperature of said compact to a first temperature that is lower than or equal to a liquid-phase formation temperature; c2) raising the temperature of said compact from said first temperature to a second temperature that is higher than said liquid-phase formation temperature and lower than a crystallization temperature of said metal oxide particles; c3) raising the temperature of said compact from said second temperature to a third temperature that is higher than said crystallization temperature and lower than or equal to a maximum temperature; and c4) maintaining the temperature of said compact at said maximum temperature for a predetermined period of time, and in said operation c3), the temperature of said compact is maintained at said crystallization temperature for a predetermined period of time. 